A transparent thermoplastic optical polymer can be stretched to achieve a desired phase difference and used as the phase difference film (retardation film) of a TFT-LCD for enabling the light transmitted through the retardation film to have a specific phase and thus increasing the viewing angle of the TFT-LCD.
Three are three conventional methods capable of producing an optical film by stretching the film in a direction normal to the film surface which are described as following:    (1) The first method, as described in the Japanese Patent Kokai (Laid-Open) 5-157911, the U.S. Pat. No. 5,472,538, the U.S. Patent Application Publication US20020063819A1, and the U.S. Patent Application Publication US20020034596A1, stretches an optical sheet constituted by a retardation film and another polymer film such that the retardation film is being stretched in the Z-axis since the shrinkage ratios of different films are different. In this method, however, the axial extension coefficients of each layer have to match with each other and the extending parameters have to be precisely controlled during the extension process. In this regard, the manufacturing cost is relatively high because of the complicated manufacturing process.    (2) The second method, as described in the Japan Patents Kokai (Laid-Open) 3-24502, 3-85519, 3-109508, 4-422-2, and 4-56802, uses an uniaxially stretched polymer optical film with positive intrinsic birefringence attached with an uniaxially stretched polymer optical film with negative intrinsic birefringence to achieve the equivalent effect similar to a Z-axis stretched film. However, the resulting laminate will have worse transmission rate, and the scopes of the application of the method are limited due to the thicker film and the accompanied problems in the optical axis.    (3) The third method, as described in the Japan Patents Kokai (Laid-Open) 2-285303, 5-157911, 7-128523, and 8-248202, plates an optical film with metal or other materials such that the optical film is extended in the Z-axis by applying an external electric field. However, not only the cost is high, the plated conducting metal has to be removed after the stretching.
Accordingly, the conventional methods for producing optical film have at least the disadvantages as following:    1. Supplemental materials are needed in the conventional methods, such as adhering another layer of polymer film or plating a conducting metal, before stretching the optical film along the Z-axis, such that the manufacturing cost is relatively higher and the procedure is more complicated.    2. Axial extension coefficients of each layer have to match with each other in the conventional methods and, the convention methods having too many variables during the stretching process such that it is difficult to control the quality of the stretch optical film precisely.    3. Supplemental materials have to be removed after the stretching process in the conventional methods, which the yield of the LCD panel after attaching the optical film is less than expected causing by the deformation of the optical film in the removal process or the residues of the supplemental materials on the optical film.    4. The optical film possesses residual strain after the stretching processing in the conventional methods that the release of the residual stain will cause incorrect size.